1. Field of the Invention
This invention relates to sealing ("stopping-off") ducts or pipes for example during repair of maintenance operations. The most common example of a duct or pipe on which this is done is a distribution duct for main gas supplies.
We are primarily concerned in this invention with the stopping-off of large ducts, say up to about 120 cm diameter, where the pressure to be resisted may be comparatively high, perhaps up to about 2 bar.
2. Description of the Prior Art
To stop-off systems which are working at that sort of pressure, primary sealing is achieved by a so-called iris-stop system. In this, an expansible disc is introduced into the duct through a hole in its wall and opened out to occupy most of its cross-section. A so-called primary bag is introduced into the duct at the upstream side of the disc. Once it is positioned inside the duct it is inflated and thereby forms a seal across the duct. The end of the bag remote from its point of inflation is supported by the disc. This therefore both prevents sliding of the bag along the duct under the high pressure encountered and reduces the general stresses on the primary bag.
A secondary bag is introduced into the duct downstream of the primary bag. The secondary bag acts as a further seal for gas seeping past the first bag. It is also desirable for the secondary bag to act as a safety device, maintaining the seal on the duct if the primary bag fails. The secondary bags currently in use do not perform this back-up safety function very well. Because it is a back-up system the secondary bag has to be designed to meet pressure differentials greater than those which were withstood by the primary system and to meet them in conditions where there has been a sudden collapse of the primary bag, which can result in a substantial shock effect. Because it has to meet anticipated higher pressures it will itself have to be inflated to substantial pressures if it is to seal effectively and so it will, even while the primary bag is still acting, be subjected to considerable stress.
In order to provide some support for the secondary bag, a support member (usually a steel tube) is inserted into the duct through the same hole as is used for insertion of the bag. The support member is placed downstream of the bag. The bag is normally inflated through its downstream end, and the pipe carrying the inflation fluid normally runs from the hole in the duct to the inflation point on the bag through the support member. This support member is much less effective than the iris disc supporting the primary bag, but it is a much simpler structure and does not require a separate insertion hole in the duct, while providing at least a degree of support for the secondary bag. The inflation fluid is in practice always compressed air or another gas, nitrogen is often used.